The present invention relates to a forming fabric for use in a papermaking machine having two warp layers and three weft layers, and more particularly to a forming fabric in which a warp layer and a weft layer defining together a paper-web supporting surface of the fabric are constructed separately from or independently of a warp layer and a weft layer defining together a wear-side (i.e. under-side) surface of the fabric.
Conventional well-known double-layer fabrics now in use in paper-making machines have two layers of synthetic weft threads disposed one above the other and a layer of synthetic warp threads interconnecting said weft threads. One of such prior art fabrics is disclosed in U.S. Pat. No. 4,071,050. In such construction of the fabrics, the warp threads are interwoven with the weft threads of an upper weft layer to form a paperweb supporting surface of the fabric, and the same warp threads are also interwoven with the weft threads of a lower weft layer to form a wear-side (i.e. underside) of the fabric. Accordingly, in order to improve the quality of paper sheets formed on the fabric, particularly to improve the "wire-mark characteristics" of the fabric, it is necessary to make the warp thread diameter smaller or finer, and on the other hand, in order to improve the wear-resistance or abrasion-resistance of the fabric, it is necessary to make the diameter larger or thicker.
This is also true even when the under-side knuckles of the weft threads of the lower weft layer are positioned outside the under-side knuckles of the warp threads so that the weft threads knuckles are subject to wear prior to the warp threads knuckles, as is the case in what are generally referred as "weft runner type" fabrics. In other words, in "weft runner type" fabrics, the lower weft threads begin to wear prior to warp threads. However, after the weft threads have been worn to a certain degree, the warp threads will also begin to wear and eventually will be worn through and then broken by the tension exerted on the fabric by the driving rolls of the paper-making machine. This leads to an extremely dangerous operation of the fabric during use, causing the fabric to be suddenly broken throughout the entire width thereof. Thus, conventional double-layer fabrics for use in paper-making machines have two incompatible requirements. Namely, when smaller diameter warp threads are employed to improve the paper-quality or particularly the "wire-mark characteristics" of the fabric, the wear resistance of the fabric is reduced and, inversely when larger diameter warp threads are employed to increase the wear resistance of the fabric, the "wire-mark characteristics" of the fabric will be deteriorated, leading to an unacceptable paper quality.
Heretofore, there have been various proposals in order to remove the above-described problem. For example, there are prior-art publications such as Japanese Public Disclosure No. 55-12892, Japanese Public Disclosure No. 50-88307, and Japanese Patent Publication No. 40-15842. Such prior techniques, however, do not sufficiently overcome the above-described problem, and most importantly, they have serious disadvantages as will be explained below in detail.
First, Japanese Public Disclosure No. 55-12892 discloses a forming fabric for use in papermaking machines which comprises a first set of warp threads and a first set of weft threads which are interwoven to form a first complete weave to define a paper-web supporting surface, and a second set of warp threads and a second set of weft threads which are interwoven to form a second complete weave to define the wear-side of the fabric, characterized in that separate binder weft threads are used to interconnect the first weave and the second weave, which binder weft threads are positioned between the two weaves and interwoven with threads from the first and the second sets of warp threads. In such construction of the fabric, since warp threads are interconnected with associated binder weft threads at a predetermined interval or pitch, the paper web supporting surface of the first complete weave tends to have an uneven pattern above or over interconnection points of threads from the first sets of warp threads with associated binder wefts. In other words, since the binder weft threads extend substantially straight between the two weaves, at interconnection points of warp threads from the first complete weave with the binder weft threads, the amount of crimp (formed during weaving) of the warp threads and that of remainder warp threads not interconnected with the binder weft threads are different, resulting in irregular patterns at the points (i.e. unevenness) different from the normal weave patterns of the first complete weave, causing "wire-mark" on the paper sheets formed on the fabric. Further, as stated above, in this fabric, the first complete weave and the second complete weave are interconnected by means of the binder weft threads and particular warp threads selected from the first and the second sets of warp threads of the two complete weaves, the selected warp threads being interconnected with the binder weft threads. Therefore, as the fabric travels around a plurality of rolls of a papermaking machine, the first complete weave is gradually displaced forwardly relative to the second complete weave, as a result of which the selected warp threads and/or the binder weft threads will be broken or cut at their interconnecting points.
Second, Japanese Public Disclosure No. 50-88307 discloses a forming fabric for use in a papermaking machine which comprises a first set of warp threads and a first set of weft threads which are interwoven to form a first complete weave, and a second set of warp threads and a second set of weft threads which are interwoven to form a second complete weave, threads from the first set of weft threads and threads from the second set of weft threads being interconnected by means of binder warp threads. For the same reason described above in connection with Japanese Public Disclosure No. 55-12892, the binder warp threads will be eventually broken, resulting in a shorter effective life of the fabric.
Third, Japanese Patent Publication No. 40-15842 discloses a fabric for use in a papermaking machine which comprises a first set of warp threads and a first set of weft threads which are interwoven to form a first complete weave, and a second set of warp threads and a second set of weft threads which are interwoven to form a second complete weave, a part of the first set of warp threads (i.e. binder warp threads) being interconnected with selected threads from the second set of weft threads. In this fabric, since the binder warp threads are interconnected with selected threads from the second set of weft threads at a predetermined interval or pitch, at these interconnection points the paper-web supporting surface of the first complete weave becomes uneven, causing "wire-mark" on paper sheets conveyed by the fabric. Furthermore, for the reason described above, the first complete weave is gradually displaced forwardly relative to the second complete weave, as a result of which, the binder warp threads will eventually be broken or cut at their interconnection points.
The present inventor has found that the above described disadvantages result from the fact that two or three complete weaves are interconnected by means of binder threads. The inventor has also found that the disadvantages described above cannot be removed through such prior art fabrics, and has invented novel forming fabrics consisting of only a single complete weave having two warp layers and three weft layers, which fabrics are capable of removing such disadvantages.